If the gaseous bubble moves in water which contains surfactant substances, other phenomena, which change its speed considerably, intervene. Owing to their structural features, these substances have the property of distributing themselves spontaneously at the water/air interphase in concentrations which are far higher than those which occur in the mass of the water, giving rise to the formation of an adsorption film which is generally monomolecular (i.e. of the thickness of one molecule of the substance adsorbed). In the case of rising gaseous bubbles, adsorption of a surfactant substance on the surface of the bubble gives rise to a film which, depending on the mass concentration of the rising path and the surface activity of the product, enriches the surface of the bubble of surfactant molecules such as to confer rigidity on the surface of the bubble. The transmission of quantities of motion to the gases (or most commonly to the fluid contained in the bubble) is prevented in this case, and friction on the surface of the bubble increases as the surface concentration increases, until that of a typical solid structure is obtained. In other words, by adsorbing the surfactant material on its surface, the bubble acquires the same features as a solid body, and therefore follows Stokes' law, which governs the motion of a solid sphere which is left free to move in a fluid of a different density.
By measuring the decrease in speed of rising of a bubble in aqueous means, the above-described phenomena as a whole permits detection of the presence of surfactants, even in the form of traces. In fact, the phenomenon of adsorption can increase up to thousands of times the difference of concentration between the surfactant molecules distributed on the water/air interphase, compared with that of the same molecules in the mass of aqueous solution. Consequently, even for small concentrations of mass of surfactant substance, considerable changes occur in the surface features of the bubble, with a final increase in rigidity of the interphase surface. Quantitatively, this phenomenon can be evaluated on the basis of the surface viscoelasticity of the adsorption film.
Adsorption is not an instantaneous phenomenon, and in order to achieve the excess of surface concentration in thermodynamic balance with the concentration of the surfactant in the mass of the water, the bubble requires a specific time equivalent to a specific path of rising for a specific concentration. The speed of rising of the bubbles also depends greatly on the dimensions of the latter; for bubbles of 2-3 mm in diameter this is approximately 30 cm per second in pure water at ambient temperature. For bubbles of this diameter, a path of several meters is required before the conditions of saturation of the respective adsorption surface are obtained. This last aspect is very important because the phenomenon of adsorption, and still more the corresponding surface viscoelastic properties, reach constant values upon full saturation of the adsorption surface. In practice, the linearity of the aforementioned phenomena is also achieved only for surface concentrations of the bubble; for the first 50 cm of path, it is virtually linear even for limit mass concentrations of 1 mg/l of surfactant, with surface activity comparable to those commonly used as detergents.
In addition to detecting the surfactants present, it is thus also possible to obtain information on their concentration. In any case, when an empirical calibration curve is plotted with a surfactant which has surface expansion properties of the same type as those of the aqueous solutions being examined, information can be obtained concerning the mass concentration of higher concentrations as well.